I have trouble deciding what accidentals to put in the key signature for pieces in dorian, phrygian, lydian or mixolydian modes. There seem to be tradeoffs associated with each choice.

For example, when writing in G mixolydian, setting an empty key signature makes it look like C major or A minor. Having the F# in the key signature forces me to put accidentals all over the piece to mark F naturals.

I prefer to use (in this example) the key signature of G major and natural signs on every F, especially when writing for choir. Singers with varying reading skills will often unconsciously assume that if the piece has G as the tonic, then it is in G major. The natural sign on every F will help them remember there is something special about it.

The music itself should clarify any confusion about key signature. How do you distinguish between C-major and A-minor? By looking at the music: pieces in A-minor have their tonal center obviously at A rather than C. If your tonal center is clearly G but all the F's are naturals, then any musician will be able to tell that it's in G-mixolydian.
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Alex BassonMay 25 '11 at 16:35

It is indeed so. I feel that I can recognise the mode, but not everyone can. And as I mentioned, some people tend to miss that and fall back automatically to G-major.
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GauthierMay 26 '11 at 5:47

And to extend the question, how would you then show B-super locrian in the key signature? Only Eb, with no Bb?
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GauthierMay 26 '11 at 5:50

I would chose the C major key signature. As somebody already said, a musician should know that the key signature is only used to simplify the process of reading and writing. For instance, Heller sometimes changes the key signature when he modulates to a different tonality.
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VictorDec 20 '11 at 10:28

Is it allowed to just scribble at the top "it's in G mixolydian, dummy!"
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Mr. BoyJan 26 at 11:43

9 Answers
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The convention generally follows that which we see for minor key signatures. There is not a 1 to 1 relationship of key signature to root, rather, the key signature is there to tell us what notes exist in the scale. Then, we use the music itself to figure out where the root is.

If you were writing in D phrygian, for example, would you have two sharps in the key signature and then naturalize all Fs and Cs while flatting the Bs and Es? I should hope not, that would be confusing as all heck as far as I'm concerned. You would write two flats and be done with it.

And yes, it's true, writing a natural key signature for G mixolydian could easily be confused for C major. OR for A minor. OR for F lydian, D dorian, or B locrian. We already have this convention for major and minor (i.e. Ionian and Aeolian), it's just that most people don't realize there are other possibilities. When they start seeing that in the music, they'll realize soon enough.

Edit

The "church modes" as we know them, are the ones with the most background in western tonality, so I consider this convention to apply primarily to them. When you're talking about scales like super locrian or lydian dominant, there exists far less precedent. These most commonly occur in jazz music, where the key signature (if any) is most likely going to be a simple representation of overall major or minor key. The key signature matters less in this situation, since the scale/mode is going to be changing with the chords every bar or so anyway.

Similarly, when you have scales that are less common in western common practice, like the Arabic scale, Ukrainian minor, what have you; there is little history of these being notated according to western traditional rules. In many cases the musical tradition would not be notated, or there would be an alternate system of notation.

In contrast, the common usage of modes like mixolydian or dorian would be in a harmonically inactive setting. This lends itself to using one key signature for the entirety of the piece. If you're trying to notate non-western-traditional music, feel free to make up your own rules, but if you're using these types of scales in a jazz or highly harmonically active context, your key signature is probably not going to be directly related (should it exist at all).

How about modes that are none of the above? For example super locrian? I do agree that I prefer having an empty key signature for G mixolydian, I changed this in songs where I realised that the low 7th went unnoticed. It's easy to associate dorian and phrygian to minor, and mixolydian and lydian to major. To follow my reasoning, I'd write D phrygian with D minor key signature and add Eb as accidentals. But as a reader I'd prefer two flats in the key signature, just as you said.
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GauthierMay 26 '11 at 5:57

Nice edit, I would like to upvote it one more time :) . I thought of one more case worth a thought. The minor harmonic scale is very common in western music, yet the major seventh is noted as an accidental (there is no other way if you want to use the common key signatures, with signs in the right order and without omission). This would be in favor of having one sharp in the key signature of G mixolydian. If you do it for harmonic minor, why not for the other modes...
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GauthierMay 27 '11 at 7:53

Writing an entire piece based on the super locrian mode, I might take it as a challenge :)
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GauthierMay 27 '11 at 7:54

@Gauthier I'm not sure I follow how that is in favor of G mixolydian with one sharp, since modal signatures do follow the traditional order of sharps/flats, but I'll give you that it's definitely an exception to the rule--we don't think of minor harmony as "modal," so the leading tone needs to exist in many cases. Traditional western notation is imperfect, that's for sure, but I think we're stuck with it!
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NReilingh♦May 27 '11 at 12:20

I've played a lot of Eastern European music and often the minor keys will show the mode in the key signature e.g G minor has F sharp and B flat in the key signature so no accidentals needed all over the place.

Another is a mode based on E that only has a G sharp in the key signature.

It may not be a familiar to a newer musician, but I find once people know about it, they're fine.

Interesting, both sharps and flats in the same key signature! I wonder what notation programs can do it.
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GauthierMay 26 '11 at 6:02

The mode based on E with a G sharp sounds like A harmonic minor, only based on E.
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GauthierMay 26 '11 at 6:02

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Re: I wonder what notation programs can do it - I'm pretty sure Sibelius / Finale and those newer notation programs deal with it fine.
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nigglesMay 26 '11 at 6:24

Re: The mode based on E with a G sharp sounds like A harmonic minor - E is the dominant chord, A minor the 5th, but the major third pulls it strongly to E despite the rest of the normally sharp notes being lowered by a semitone. European music has some fun conventions like H = B and B = B flat when it comes to chords :-)
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nigglesMay 26 '11 at 6:27

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To be more precise, the H and B convention of being B and Bb is in European countries with German based languages. Germany and Scandinavia, for example. In latin-European countries such as Italy and France, fixed-do is used and the notes are called do re mi fa sol la si/ti (according to wikipedia, that is the case "In the major Romance and Slavic languages"). I just wanted to point out that there are differences within Europe :)
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GauthierMay 26 '11 at 6:43

Most music I've seen that uses modes other than major and minor just uses the key signature that requires the fewest accidentals, and I've seen people who try to read it run into both problems you mention. (I'm thinking of two different choir pieces in F# mixolydian: people thought the first one was in B major and sight-read the second one as if the key signature were actually F# major.)

I think the solution you prefer is fine, but the real solution would be to increase general awareness of modes besides major and minor. :)

Yes, that's another problem. When you come to that amount of sharps in the key signature, you don't really count them. I would just assume the tonality from the start of the piece, which is not good.
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GauthierMay 26 '11 at 6:00

I appear to be the only one (with my quick viewing of this thread) that feels that you SHOULD have accidentals. If you're in D dorian, write as if it was D minor (aka one flat) and have B naturals as accidentals. That lets your performer know that you're in a D minor mode, and looking at accidentals clarifies it.

Because it's so easy to divide the modes into Major Modes and Minor Modes, I'd stick to the key signatures of either the Minor or Major key of your scale, with appropriate accidentals. Accidentals do not inherently mean "by accident", nor does a piece require a key signature to be in a key (oh gods Poulenc's Oboe Sonata?). Consistent accidentals give just as much information as a key signature, and sometimes more.

I think the main problem that previous posters had with using existing key signatures and accidentals, was that people used examples of Minor Modes with Major key signatures. That's just silly guys! :-P

When I was a beginning musician I wanted the key signature that told me what notes to play, with the fewest accidentals written in the line. When I got a little more advanced I realized that knowing the "real" key/mode is important for understanding harmonic properties. This matters if you need to know where your part is in the blend (e.g. am I on the 3rd? the 5th? are you going to make me look at the bass line to find out?) or if anybody is going to be improvising. In your example, it is the difference between people thinking the piece is in C and knowing that it is in some flavor of G (though not ionian).

So unless you are writing a line for a single voice/instrument that will not be accompanied in any way, I would notate it per mode and use the accidentals.

Monica -- your answer brought back memories. I remember playing pieces in middle school where there would be a pair of key changes (say from C-major to G-major and back) where in the 1-sharp section I didn't have a single F or F# written so I wondered why the composer bothered to change the key signature for me. It wasn't until later that I realized the wisdom of showing a player the surrounding key/mode.
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Michael Scott CuthbertApr 18 '13 at 2:02

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Yes, I wondered about that sort of thing too! Eventually I learned. :-) (While I learned to play music from a young age, I didn't actually learn any music theory until college -- at which point I realized how much I'd been missing out on.)
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Monica CellioApr 18 '13 at 2:06

I imagine the tuba players must have run across this often, since if you're usually just playing the bass of I, ii, IV, V then adding a sharp and switching to the dominant doesn't ever use the new sharpened 7th degree.
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Michael Scott CuthbertApr 18 '13 at 2:14

True! (I never played with brass; we had a marching band only, and I played piano. :-) )
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Monica CellioApr 18 '13 at 2:18

The term "key signature" used in English do not translate exatly the meaning of what it is.
In all other languagues, the literal translation would be "clef armature". So a clef armature is not meant to represent a musical key, but to simplify the process of reading music. It works fine for tonal music, and often we can deduce the key of a piece through it.
In contemporary music, after the atonal, dodecafonic and serial schools, this notation is becoming more and more deprecated.

For modal music, as you ask, you should use the key signature/clef armature relative to the pure mode. For example, two sharps for E dorian, four flats for Eb lydian and so on. In cases where you want a complex mode, you'll need to put the extra accidentals as they occur. For example, a C augmented mixolydian would have a B flat at the clef (for C mixolydian), then a G sharp along the score. This is the simplest way for the instrumentist to read.

If you want your players to know exatly the mode of your tune or passage (usefull for guiding improvisation), you might write it down explicitly.

Edit:

Just saw this discussion, and this site for that matter. Both very nice!

Keep it simple ! Use the key signature of the 'mother key'.E.g. D Dorian would have no # or b, C Phrygian would be 4flats etc. No-one has mentioned the difference between , say, G Dorian and the Dorian of G - 2 different keys !

Just saw this discussion, and this site for that matter. Both very nice!

I always choose to write my music as much as possible to reflect the ideas that I have.

If I write a modal melody, I don't want to see any accidental sharps or flats, because there is nothing accidental about them (unless of course I construct a melody that uses a note outside the mode)

For instance, when I wrote a piece in G Lydian dominant scale (G A B C# D E F#), I used a key signature with only these two sharps (C# and F#). Then people have to find out about the key, and will then perceive all notes in that context.

The only exception to this for me would be if I am sure that the people reading and playing the music have no idea about these matters, and are inexperienced musicians. Then maybe I'd write in a G major scale (hoping they understand G as the tonic, and if not, at least seeing a familiar key) and lots of accidentals, making sure they notice these strange notes! :)

Welcome here! The leading tone in minor harmonic mode is noted as accidental. As I wrote in an earlier comment, super locrian and super lydian cannot be noted without accidentals (if you don't allow for very strange key signatures). If we accept accidentals for harmonic minor, super locrian, super lydian, and what not, then I guess we could accept it for lydian and other church modes...
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GauthierDec 20 '11 at 8:01